For example, in a boiling-water reactor (BWR), each control rod is placed in a gap between four fuel assemblies arranged in a square lattice, and is inserted or withdrawn. Accordingly, the control rod is formed to be a cross shape, and is shaped in such a way as to have wing portions in four directions. Each wing portion contains a neutron absorbing member.
The wing portion has, as a wing surface structure member, a sheath whose cross-section is deep U-shaped, and the sheath contains the neutron absorbing member therein. In a conventional control rod, a metal material, such as stainless steel, is used. Materials containing such elements having large neutron cross section as boron and hafnium (Hf) are used as neutron absorbing material.
For example, in some control rods using boron, boron carbide (B4C) is used as a neutron absorbing material. A neutron absorption rod may be formed by encapsulating powder or pellets of B4C in an rod-shaped absorber made of stainless steel. The neutron absorption rod may be disposed in the sheath.
In some control rods using Hf, the sheath stores plates or flattened tubes made of Hf metal or an Hf alloy as neutron absorbing members therein.
The control rods are inserted or drawn via a lower portion of a reactor core by a control rod drive mechanism by means of hydraulic driving or electric driving. In an emergency, the control rod is inserted by the pressure of gas stored in an accumulator or the like.
The control rod drive mechanism of BWR is located below the control rod. Therefore, in order to secure the margin of driving power for insertion of the control rod drive mechanism, lightweight control rods are desirable.
Considering future trends such as increasing enrichment of fuel for high burnup of the fuels, worth of the control rods may be increased. Such measures of increasing the amount of Hf to achieve this causes increase of the weight of the control rods because of the high density of Hf, which is 13.3 g/cm2. Therefore, such a control rods could not easily be applied to an existing reactor due to weight restrictions.
Some control rods use Hf stored in stainless steel sheaths. Then, corrosion advances between different metals, or between stainless steel and Hf, and the sliding friction between Hf and the stainless-steel sheath becomes higher due to corrosion products. Thus, stress is applied to the stainless-steel sheath due to a difference between expansion of Hf (irradiation growth or thermal expansion) and expansion of the stainless-steel sheaths, and some stainless-steel sheaths have been damaged.
In the case of BWR, fuel assemblies are surrounded by channel boxes made of a zirconium (Zr) alloy. However, there are reports of the shadow corrosion phenomenon, a phenomenon of significant corrosion on a surface facing a conventional stainless-steel sheath control rod (different metals).
When a critical event, such as loss of power, occurs due to a large-scale natural disaster, the insertion of control rods can shut a nuclear reactor. However, if the core cooling system fails to actuate, decay heat of fission products in the fuel or like may cause temperature rise of the nuclear reactor.
In the reactor internal structure of BWR, metallic materials have been mainly used, such as a Zr alloy for fuel rods and channel boxes and stainless steel for control rods. These metallic materials are oxidized by high-temperature steam through a metal-water reaction, to generate hydrogen gas. As the generation of hydrogen advances, the concentration of hydrogen inside a containment vessel increases. Under some conditions, the integrity of the containment vessel may be damaged by hydrogen combustion.
If the cooling of the nuclear reactor is not performed for an even longer time, the reactor core is further heated by decay heat, possibly resulting in an abnormally high temperature. The melting point of stainless steel, which is a structural material of the conventional control rods, is about 1,400 degrees Celsius. Meanwhile, fuel cladding tubes and the channel boxes are made of the Zr alloy; the melting point of Zr is about 1,850 degrees Celsius.
If the temperature of the reactor core continues rising, the controls rods could melt and fall down from the reactor core while the fuel assemblies do not melt. If such an event occurs and cooling water system becomes available, cooling water is injected into the reactor core without control rods inside. In this case, the water works as neutron moderator, and unintentionally might cause criticality of the reactor core.
Accordingly, the structural material keeping the structure of the control rods is desirable to be heat-resistant enough to maintain the shape at least during a period in which the fuel assemblies and channel boxes maintain their configurations.
As for the Hf control rods, in order to prevent the damage caused by thermal stress or an irradiation growth difference between different metals, or between the Hf neutron absorbing member and the stainless-steal sheath, as well as to prevent shadow corrosion by different-metal corrosion between the channel box and the stainless-steal sheath, structures in which Zr is used as the sheath of the control rod or a Hf—Zr alloy is used as a wing portion are proposed.
As an example using a high-temperature material for the control rods of the nuclear reactor, use of carbon/carbon composite (C/C composite) material and silicon carbide/silicon carbide composite (SiC/SiC composite) material is proposed for a control rods of a high-temperature gas reactor that is different from a control rod of the BWR.
The control rods in the boiling water reactor are constantly in contact with high-temperature water, that is different from the high-temperature gas reactor. Carbon fiber (C-fiber) or the like is oxidized by the water, resulting in a significant decrease in strength.
AS an example that the SiC is used as general structural material of the nuclear reactor, application of SiC material containing 11B isotope that is stable under irradiation of neutron is proposed. In the case of application to the control rods of the light water reactor, 10B which has a large neutron absorption cross section and causes an effect of increasing neutron absorption is preferred to be contained on some level to the extent without bad influence.